In down hole well logging devices radioactive phenomena are utilized to measure various properties of the formations adjacent to the well borehole. Typical techniques utilize gamma ray or neutron irradiation procedures and hence require detectors. The output signal of such detectors is a random mix of pulses of different spacing and different amplitude. The nuclear events need to be accurately captured and reproduced to obtain accurate analysis.
Typically, the output of a nuclear detector is integrated to recover the entire energy of each pulse of the nuclear detector. This results in a pulse waveform that rises relatively rapidly but collapses relatively slowly. This wave form is typically linearly amplified to a level sufficient to enable the signal to be input to a discriminator circuit. The discriminator has an adjustable threshold value. Thus, signal levels above the threshold are significant and those which are below the threshold value are not significant. Because the nuclear events are random in amplitude as well as frequency, the setting of the threshold value in the discriminator may very well modify the output signal from the discriminator.
While the events may occur in a random distribution, as the number of events in a given interval increases, there is an increasing probability that first and second nuclear events will overlap before the system including the linear amplifier and discriminator has finished processing the prior event. This provides overlapping events wherein the signal processing is dependent on the relative amplitude and spacing between the events. Accordingly, dead-time is defined as that interval wherein the system is unable to respond to the trailing event because it is too closely spaced to the preceeding event. This minimum time or dead-time describes a limitation on the data processing capacity of the system. Dead-time exists in all the components of the signal processing circuitry. As will be understood, signal dead-time is cumulative for the system. As the average rate of the events occuring at the nuclear detector approaches a rate corresponding to the reciprocal of system dead-time, the ability of the system to count individual events decreases significantly. Corrections based upon system dead-time can be implemented, but as such corrections increase system accuracy is reduced. A reduction in dead-time reduces the corrections which might otherwise be necessary and thereby improves system accuracy.
This system utilizes a reduced dead-time signal processing circuit involving a charging capacitor at the output of a differentiating circuit. Thus, the event is first detected, input to an integrator circuit, that signal is then amplified, and the amplified output is then differentiated. The differentiated signal is input to a series capacitor. On the output side of the capacitor, the fall time is made much more rapid than at the input side. This is accomplished by connecting appropriately biased diodes to the output side of the capacitor with a view of changing the capacitor output fall time. Thus, this change in slope on the back side of a nuclear event coupled through the capacitor enables the next following nuclear event to be separated from the prior event. This more rapid change in the fall time on the back side of a particular pulse shortens the dead-time and enables the next pulse to be observed sooner. That is, the trailing pulse, even if significantly smaller, is more readily observed because the dead-time has been modified to enable the prior larger or smaller pulse to clear the capacitor as a result of the modified fall time (the trailing edge) of the prior pulse.